This invention relates to a driving device (a linear actuator) and, in particular, to a position detecting device for detecting a position of a movable portion.
Previously, linear actuators (driving devices) using electro-mechanical transducers such as piezoelectric elements, electrostrictive elements, magnetrostrictive elements, or the like are used as auto-focus actuators or zoom actuators for use in cameras. The cameras may be mobile cameras mounted on cellular mobile phones or the like.
By way of illustration, Japanese Patent No. 2633066 (JP-B-2633066) (which will be also called a first patent document), which corresponds to U.S. Pat. No. 5,225,941, discloses a driving device comprising a driving rod frictionally engaged with a lens barrel, a piezoelectric element disposed in contact with the driving rod, and a leaf spring for bringing the driving rod into frictional engagement with the lens barrel. That is, the driving rod is bonded to an end of the piezoelectric element in an expansion direction. The lens barrel is movably supported to the driving rod. The leaf spring produces friction between the driving rod and the lens barrel. In the driving device disclosed in JP-B 2633066, a voltage is applied to the piezoelectric element so as to make a speed of expansion of the piezoelectric element different from a speed of contraction thereof. The driving device disclosed in JP-B-2633066 comprises a lens-barrel position detector. The lens-barrel-position detector comprises a U-shaped slidable contact piece secured to the underside of an arm of the lens barrel and a fixed detecting plate mounted on a fixed member which is disposed below the path of movement of the lens barrel.
However, the lens-barrel-position detector disclosed in the first patent document is disadvantageous in that its slidable portion wears because the U-shaped slidable contact piece in slidably contact with the fixed member.
In addition, Japanese Patent No. 3218851 (JP-B-3218851) (which will be also called a second patent document), which corresponds to U.S. Pat. No. 5,589,723, discloses a driving apparatus comprising a piezoelectric element, a driving member (a driving shaft), coupled to the piezoelectric element, for extending in an expansion direction of the piezoelectric element, and a driven member (a moving member, a lens barrel) having a friction member frictionally coupled to the driving member (the driving shaft). The driving apparatus in JP-B-3218851 drives the lens barrel by devising a driving signal applied to the piezoelectric element.
The second patent document neither discloses nor teaches a position detecting device.
Japanese Unexamined Patent Application Publication No. H09-191665 (JP-A-09-191665) (which will be also called a third patent document), which corresponds to U.S. Pat. No. 5,890,391, discloses a linear drive mechanism using an electromechanical conversion element which is insensitive to elastic deformation of a constituent member. The linear driving mechanism disclosed in JP-A-09-191665 comprises the electromechanical conversion element, a working member which is fixedly connected to the electromechanical conversion element and which displaces with the electromechanical conversion element, a driving member frictionally coupled to the working member, a driven member coupled to the driving member, and a driving pulse generating arrangement for causing expansion/contraction displacement to the electromechanical conversion element. In JP-A-09-191665, an internal barrel is integratedly and fixedly attached to a fixed lens barrel. The internal barrel has extensions which are formed in a radial direction on part of the internal barrel. A drive shaft (a moving portion) arranged parallel to an optical axis is supported by bearings of the extensions so as to be movable in the optical axis direction. A piezoelectric element (the electromechanical conversion element) is disposed between the working member (a vibration friction portion) and the extension of the internal barrel. The top half of the working member touches the drive shaft and a pad is installed on the bottom half of the working member and exerts a force toward the drive shaft via a spring, such that the pad touches the bottom half of the working member. The working member and the pad are friction-bonded to the drive shaft via a suitable friction force. By generating expansion/compression displacements having different rates in the thickness direction of the piezoelectric element, the working member is displaced, and the drive shaft is driven by friction contact with working member together with a lens holding frame serving as the driven member.
The linear drive mechanism disclosed in the third patent document is provided with a position sensor of a ferromagnetic thin-film reluctance element type (hereinafter referred to as “MR sensor”) for detecting a moving distance of the lens. The moving distance of the lens is also a moving distance of the lens holding frame (a guide shaft disposed in parallel with the optical axis) A magnetic reluctance element comprising the MR sensor is disposed in the internal barrel, and magnetic rods of north and south magnetic poles are arranged at predetermined intervals on the guide shaft. That is, the MR sensor disclosed in the third patent document detects the moving distance of the lens holding frame in cooperation with the magnetic reluctance element.
Japanese Unexamined Patent Application Publication No. 2006-113874 (JP-A-2006-113874) (which will be also called a fourth patent document) discloses a piezoelectric actuator comprising a position sensor. The piezoelectric actuator disclosed in JP-A-2006-113874 comprises a piezoelectric element (a electromechanical transducer) having one end which is fixed to a fixed portion (a stationary member) by means of adhesive or the like, a rod-shaped drive member (a vibration friction portion) fixed to another end of the piezoelectric element by means of adhesive or the like, and a movable member (a moving portion) which is movably frictionally held on the drive member. The movable member comprises a friction holding portion which engaged on the drive member by a friction force and a lens holder for holding a lens that is coupled to the friction holding portion. The position sensor comprises a magnetic field generating member mounted on the movable member and a magnetic field detecting member fixed on the fixed portion so as to be opposed to the magnetic field generating member. A Hall-effect device, a magneto-resistive (MR) element or the like is suitable for the magnetic field detecting member.
A method of using, as a detection element, the Hall-effect element or the MR element (the MR sensor) in the manner of the third and the fourth patent documents is disadvantageous to miniaturization and cost. This is because it is necessary to mount a magnetic substance bearing magnetism to the moving portion or the fixed portion.
In addition, Japanese Unexamined Patent Application Publication No. 2003-185406 (JP-A-2003-185406) (which will be also called a fifth patent document) discloses a position detecting device of non-contact type which detects a position of a moving member on the basis of capacitance between a fixed electrode and the moving member of the actuator. In the position detecting device disclosed in JP-A-2003-185406, a piezoelectric element (an Electro-mechanical transducer) has an end fixed to a frame (a stationary member) and another end fixed to a drive shaft (a vibration friction portion) with which a moving member (a moving portion) is coupled frictionally. A detecting member is arranged in non-contact with and parallel to the moving member in a moving direction. The detecting member has an electrode with rugged regions formed on a surface opposite to the moving member and is spaced at a distance from the moving member so as to form a capacitor with a capacitance. A drive circuit produces drive pulses which are applied on the piezoelectric element, make electric current flow through the electrode capacitively coupled to the moving member. The amount of the electric current is detected by a detecting circuit and is supplied to a control circuit. When the moving member moves on the electrode, the amount of the electric current is increased ore decreased in response to the shape of the rugged regions of the electrode. It is therefore possible to detect the position of the moving member based on the electric current.
In the manner as described above, the fifth patent document discloses the position detecting device of non-contact type which detects the position of the moving member on the basis of capacitance between the fixed electrode and the moving member. However, the position detecting device disclosed in the fifth patent document is disadvantageous in that power is consumed because it is necessary to always flow the electric current in the electrode. In addition, the position detecting device disclosed in the fifth patent document is also disadvantageous in that there are limitations on selecting material because the moving member must be made of a conductive material.
Furthermore, Japanese Unexamined Patent Application Publication No. 2005-147955 (JP-A-2005-147955) (which will be also called a sixth patent document) discloses a position detecting device which is capable of detecting a current position of a light-shielding plate over a range of the breadth of a slit or larger. The light-shielding plate, which is capable of reciprocating motion in predetermined directions passing through the gap of a photo-interrupter, shields light projected from a light-emitting body toward a light-receiving body. The light-shielding plate changes in width extending in a direction orthogonal to the predetermined directions. The light-shielding plate has a shape which proportionally increases a light-shielding area covering the slit of the photo-interrupter in accordance with the amount of the movement of the light-shielding plate.
However, the sixth patent document merely discloses the position detecting device and the sixth patent document neither discloses nor teaches how to dispose the position detecting device in a case where it is applied to the driving device.
Japanese Unexamined Patent Application Publication No. H06-174999 (JP-A-06-174999) (which will be also called a seventh patent document) discloses a lens initial position detecting device using a photo-interrupter that is capable of accurately detecting the initial position of the lens enable to move in lens barrel. The lens initial position detecting device disclosed in JP-A-06-174999 comprises the photo-interrupter mounted on one of the lens barrel and a lens frame and a light-shielding plate mounted on the other of the lens barrel and the lens frame so that the light-shielding plate passes through between a light-emitting element and a light-receiving element of the photo-interrupter on moving of the lens frame.
However, the seventh patent document merely discloses the lens initial position detecting device which is capable of detecting the initial position of the lens and cannot detect a moving position (an actual position) of the lens.
Japanese Unexamined Patent Application Publication No. H09-306122 (JP-A-09-306122) (which will be also called an eighth patent document), which corresponds to U.S. Pat. No. 6,072,655, discloses a position detecting unit for a magnetic disk drive although it is not a position detecting unit for a driving device using the electro-mechanical transducer. The position detecting unit disclosed in the eighth patent document comprises a scale mounted on a carriage at a side thereof opposed to a main surface of a main frame and a photointerrupter mounted on the main frame. The scale comprises slits bored through the scale at substantially equally spaced intervals along the scale in a direction parallel to a predetermined radial direction. The photointerrupter includes a light-emitting section and a light-receiving section which are opposed to each other with the scale arranged therebetween. Such a method of detecting a position using the position detecting unit comprising an optical position detecting element such as the photointerrupter and a position information portion such as the scale is called an encoder method.
In the position detecting unit of the encoder method disclosed in the eighth patent document, the accuracy of position detection is determined by a resolution of the used optical position detecting element and a workable size of the position information portion. On the other hands, in the driving devices for the mobile cameras, a position adjustment accuracy of several micrometers to some ten micrometers is required. Therefore, in order to carry out position detection using the position detecting unit of the encoder method in the driving device mounted in the mobile camera, it is necessary to work the scale so as to form the slits or the like at intervals of several micrometers. However, such a working is extremely difficult and is not down-to-earth.